Chemical vapor deposition (CVD) involves the generation of metal halide gas at low temperatures (e.g. about 100 to 600 degrees C.), introduction of the metal halide gas into a high temperature retort (e.g. 200 to 1200 degrees C. retort temperature), and reaction of the metal halide with substrates positioned in the retort to form a coating thereon. In general, a large excess of metal halide gas is used to prevent reactant starvation in the high temperature coating retort. CVD processes typically are conducted at reduced pressure (subambient pressure). CVD apparatus and method are described in Howmet U.S. Pat. Nos. 5,261,963 and 5,263,530. Howmet U.S. Pat. No. 6,143,361 described CVD apparatus and method wherein deposition of excess metal halide reactant in the coating gas exhausted from the coating retort is reduced or eliminated to reduce retort downtime required to remove deposits from the retort exhaust system.
The CVD process can be used to codeposit Al, Si, and one or more reactive elements such as Hf, Zr, Y, Ce, La, etc. to form protective aluminide diffusion coatings on substrates such as nickel and cobalt base superalloys commonly used to cast gas turbine engine airfoils. Copending U.S. Ser. Nos. 08/197,497 and 08/197,478 disclose CVD apparatus and method to produce protective reactive element-modified aluminide diffusion coatings. U.S. Pat. No. 5,989,733 describes a protective outwardly grown, platinum-modified aluminide diffusion coating containing Si and Hf and optionally Zr, Y, Ce, and/or La formed on a nickel or cobalt base superalloy substrate by such CVD apparatus and process.
There is a need to provide improved CVD apparatus and method that are capable of producing aluminide diffusion coatings modified by inclusion of one or more other coating elements, such as for example only silicon and one or more so-called reactive elements, wherein the coatings can be produced having a more uniform coating composition, microstructure, and thickness throughout the working volume (throughout multiple coating zones) of the CVD coating apparatus. It is an object of the present invention to satisfy this need.